Why NASA Watches Airglow, the Colors of the (Upper Atmospheric) Wind

Most are familiar with beloved images of the blue marble or pale blue dot -- Earth from 18,000 and 3.7 billion miles away, respectively. But closer to home, at the boundary between Earth and space, you might encounter an unfamiliar sight. If you were to peer down on Earth from just 300 miles above the surface, near the orbit of the International Space Station, you could see vibrant swaths of red and green or purple and yellow light emanating from the upper atmosphere.

This light is airglow.

Airglow occurs when atoms and molecules in the upper atmosphere, excited by sunlight, emit light to shed their excess energy. Or, it can happen when atoms and molecules that have been ionized by sunlight collide with and capture a free electron. In both cases, they eject a particle of light -- called a photon -- in order to relax again. The phenomenon is similar to auroras, but where auroras are driven by high-energy particles originating from the solar wind, airglow is energized by ordinary, day-to-day solar radiation.

Unlike auroras, which are episodic and fleeting, airglow constantly shines throughout Earth's atmosphere, and the result is a tenuous bubble of light that closely encases our entire planet. (Auroras, on the other hand, are usually constrained to Earth's poles.) Just a tenth as bright as all the stars in the night sky, airglow is far more subdued than auroras, too dim to observe easily except in orbit or on the ground with clear, dark skies and a sensitive camera. But it's a marker nevertheless of the dynamic region where Earth meets space.

Stretching from roughly 50 to 400 miles above the surface, this region, called the ionosphere, is an electrified layer of the upper atmosphere, cooked by extreme ultraviolet radiation from the Sun until molecules break apart, giving rise to a mix of charged ions and electrons. It's neither fully Earth nor fully space, and instead, reacts to both terrestrial weather -- the weather we experience on Earth -- rippling up from below and solar energy streaming in from above, forming a complex space weather system of its own. Turbulence in this ever-changing sea of charged particles can manifest as disruptions that interfere with orbiting satellites or communication and navigation signals used to guide airplanes, ships and self-driving cars.

Understanding the ionosphere's extreme variability is tricky because it requires disentangling interactions between the different factors at play -- interactions of which we don't have a clear picture. That's where airglow comes in.

"Each atmospheric gas has its own favored airglow color depending on the gas, altitude region, and excitation process, so you can use airglow to study different layers of the atmosphere," said Doug Rowland, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We're not studying airglow per se, but using it as a diagnostic."

Airglow carries information on the upper atmosphere's temperature, density, and composition, but it also helps us trace how particles move through the region itself. Vast, high-altitude winds sweep through the ionosphere, pushing its contents around the globe -- and airglow's subtle dance follows their lead, highlighting global patterns.

Two NASA missions take advantage of our planet's natural glow to study the upper atmosphere: ICON -- short for Ionospheric Connection Explorer -- and GOLD -- Global-scale Observations of the Limb and Disk. ICON focuses on how charged and neutral gases in the upper atmosphere interact, while GOLD observes what's driving change -- the Sun, Earth's magnetic field or the lower atmosphere -- in the region. By watching and imaging airglow, the two missions enable scientists to tease out how Earth's weather and space intersect, dictating the region's complex behavior.